1. Field of the Invention
The present invention relates to a cutting device for a sewing machine, having a cutting area in the vicinity of its sewing area; the cutting device including a knife and a knife guard which is movable into a protective position surrounding the cutting area; common drive means for driving the knife and the knife guard; and mechanical stop means for stopping the knife, and releasing the knife only when the knife guard is substantially in its protective position surrounding the cutting area.
2. Background Art
In one such cutting device, known from U.S. Pat. No. 4,735,160, the knife guard first moves into a position in front of the cutting place so as to protect the fingers of the operator, and then the knife carries out its cutting movement. For this purpose, the knife guard and the knife are coupled to each other by a toggle lever which is connected to a lifting cylinder hen the lifting cylinder moves outward, initially only the knife guard is driven and moved toward the cutting place. When the knife guard has assumed its position in which it protects the fingers of the operator, an interlock for the knife which has been engaged until that time is released, permitting the knife to be driven by the further outward movement of the lifting cylinder and moved to the cutting place. If the knife guard does not assume its protecting position, i.e., is not lowered in front of the cutting place, the interlock prevents the knife from being moved.
This known device is relatively large. It must be arranged as close as possible behind the sewing place, so it can only be used on sewing machines which offer a corresponding amount of space. In order to form the cut cleanly, the knife must be guided during its operation. To permit thicker materials, for instance piping or slide fasteners, to be cut after they are sewn, sufficient space must be available for them to pass between the knife and the cutting place. As a result of this, the knife guides are relatively far away from the cutting place and this causes additional forces to act on them, which makes them technically very costly.
It is furthermore known to use so-called shears as a cutting device. In accordance with the prior art, for reasons of safety, the available passage space in the open position of these shears may be so limited that the operator cannot get his or her fingers below the cutting edge, which has the disadvantage that the thickness of the material that can be worked is also limited. Safety may also be improved by controlling the shears in such a manner that automatic cutting cannot take place, thus requiring the cutting to be initiated by the operator. In the latter case, a two-hand safety operation with a manually adjustable protective device for third parties is provided. This has the disadvantage that the operator must remove both hands from the sewing material while it is being cut, and thus can no longer guide it.
U.S. Pat. No. 4,281,607 discloses a cutting device which is provided with a guard means. In this case the lowered guard actuates an electric switch means which causes the knife drive to be able to receive control signals only when the guard means is in its protecting position. However, if the electrical control device fails, the danger exists that the knife will descend even though the knife guard is not in position.